Flexible merge scheme for source/drain epitaxy regions

ABSTRACT

A method includes etching a first semiconductor fin and a second semiconductor fin to form first recesses. The first and the second semiconductor fins have a first distance. A third semiconductor fin and a fourth semiconductor fin are etched to form second recesses. The third and the fourth semiconductor fins have a second distance equal to or smaller than the first distance. An epitaxy is performed to simultaneously grow first epitaxy semiconductor regions from the first recesses and second epitaxy semiconductor regions from the second recesses. The first epitaxy semiconductor regions are merged with each other, and the second epitaxy semiconductor regions are separated from each other.

PRIORITY CLAIM AND CROSS-REFERENCE

This application is a divisional of U.S. patent application Ser. No.15/492,142, entitled “Flexible Merge Scheme for Source/Drain EpitaxyRegions,” filed on Apr. 20, 2017, which application is incorporatedherein by reference

BACKGROUND

Technological advances in IC materials and design have producedgenerations of ICs where each generation has smaller and more complexcircuits than the previous generation. In the course of IC evolution,functional density (i.e., the number of interconnected devices per chiparea) has generally increased while geometry size (i.e., the smallestcomponent (or line) that can be created using a fabrication process) hasdecreased. This scaling down process generally provides benefits byincreasing production efficiency and lowering associated costs.

Such scaling down has also increased the complexity of processing andmanufacturing ICs and, for these advances to be realized, similardevelopments in IC processing and manufacturing are needed. For example,three-dimensional transistors such as a Fin Field-Effect Transistors(FinFETs) have been introduced to replace planar transistors. Althoughexisting FinFET devices and methods of fabricating FinFET devices havebeen generally adequate for their intended purposes, they have not beenentirely satisfactory in all respects. For example, the FinFETs fordifferent circuits such as core (logic) circuits and Static RandomAccess Memory (SRAM) circuits may have different designs, and thesource/drain epitaxy regions grown from neighboring fins may need to bemerged for some circuits (such as logic circuits), and need to beseparated from each other for other circuits (such as SRAM circuits).However, to save manufacturing cost, the epitaxy for different regionsis performed simultaneously. This causes difficulty for selectivelymaking epitaxy regions merged for some circuits, and not merged forother circuits. Accordingly, the merged epitaxy regions need to betrimmed to separate the merged epitaxy regions from each other.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIGS. 1A through 10C are cross-sectional views and perspective views ofintermediate stages in the formation of Fin Field-Effect Transistors(FinFETs) in accordance with some embodiments.

FIG. 11 illustrates the exemplary layouts of a logic circuit and aStatic Random Access Memory (SRAM) circuit in accordance with someembodiments.

FIG. 12 illustrates a process flow for forming FinFETs in accordancewith some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the invention. Specificexamples of components and arrangements are described below to simplifythe present disclosure. These are, of course, merely examples and arenot intended to be limiting. For example, the formation of a firstfeature over or on a second feature in the description that follows mayinclude embodiments in which the first and second features are formed indirect contact, and may also include embodiments in which additionalfeatures may be formed between the first and second features, such thatthe first and second features may not be in direct contact. In addition,the present disclosure may repeat reference numerals and/or letters inthe various examples. This repetition is for the purpose of simplicityand clarity and does not in itself dictate a relationship between thevarious embodiments and/or configurations discussed.

Further, spatially relative terms, such as “underlying,” “below,”“lower,” “overlying,” “upper” and the like, may be used herein for easeof description to describe one element or feature's relationship toanother element(s) or feature(s) as illustrated in the figures. Thespatially relative terms are intended to encompass differentorientations of the device in use or operation in addition to theorientation depicted in the figures. The apparatus may be otherwiseoriented (rotated 90 degrees or at other orientations) and the spatiallyrelative descriptors used herein may likewise be interpretedaccordingly.

Fin Field-Effect Transistor (FinFETs) and the method of forming the sameare provided in accordance with various exemplary embodiments. Theintermediate stages in the formation of the FinFETs are illustrated. Thevariations of some embodiments are discussed. Throughout the variousviews and illustrative embodiments, like reference numbers are used todesignate like elements.

FIGS. 1A through 10C illustrate the intermediate stages in the formationof FinFETs. The steps shown in FIG. 1A through 10C are also illustratedschematically in the process flow 500 shown in FIG. 12. Each of thefigure numbers in FIGS. 1A through 10C may include letter “A,” “B,” or“C,” wherein letter “A” indicates that the respective figure illustratesa perspective view, and letter “B” indicates that the respective figureis obtained from the plane same as the vertical plane containing lineB-B in FIG. 1A, and letter “C” indicates that the respective figure isobtained from (and combined) the planes same as the vertical planescontaining lines C-C in FIG. 1A. Accordingly, the figures whose numbersinclude letter “B” show the cross-sectional views obtained from thevertical planes parallel to the lengthwise directions of gate stacks,and the figures whose numbers include letter “C” show thecross-sectional views obtained from the vertical planes parallel to thelengthwise directions of semiconductor fins, which will be discussed indetail in subsequent paragraphs.

FIG. 1A illustrates a perspective view in the formation of a structureincluding substrate 20, isolation regions 22, semiconductor strips 24between isolation regions 22, and semiconductor fins 26 over the topsurfaces of isolation regions 22. Substrate 20 is a semiconductorsubstrate, which may be a silicon substrate, a silicon carbon substrate,or a substrate formed of other semiconductor materials such as III-Vcompound semiconductor materials. Substrate 20 may be lightly doped witha p-type or an n-type impurity.

Isolation regions 22 may be, for example, Shallow Trench Isolation (STI)regions. The formation of STI regions 22 may include etchingsemiconductor substrate 20 to form trenches, and filling the trencheswith a dielectric material(s) to form STI regions 22. STI regions 22 mayinclude silicon oxide, and other dielectric materials such as nitridesmay also be used. Semiconductor fins 26 overlap the underlyingsemiconductor strips 24. The formation of semiconductor fins 26 mayinclude recessing STI regions 22, so that the portions of semiconductormaterial between the removed portions of STI regions 22 becomesemiconductor fins 26. Semiconductor fins 26 and some or substantiallyentireties of semiconductor strips 24 may be formed of silicon (with nogermanium therein) or other silicon-containing compound including, andnot limited to, silicon carbon, silicon germanium, or the like.

A plurality of parallel gate stacks 28 is formed on semiconductor fins26. Gate stacks 28 are parallel to each other, and cover portions ofsemiconductor fins 26, while leaving some other portions ofsemiconductor fins 26 uncovered. Gate stacks 28 include gate dielectrics32 on the sidewalls and the top surfaces of semiconductor fins 26, andgate electrodes 34 over gate dielectrics 32. Gate dielectrics 32 may beselected from silicon oxide, silicon nitride, gallium oxide, aluminumoxide, scandium oxide, zirconium oxide, lanthanum oxide, hafnium oxide,combinations thereof, and multi-layers thereof. Gate electrodes 34 maybe formed of a conductive material that includes polysilicon, arefractory metal, or the respective compound including, e.g.,polysilicon, Ti, W, TiAl, TaC, TaCN, TaAlC, TaAlCN, TiN, and TiW. Inother examples, gate electrodes 34 include nickel (Ni), gold (Au),copper (Cu), or the alloys thereof.

In accordance with some embodiments of the present disclosure, gatestacks 28 remain in the final FinFETs, and form the gate stacks of thefinal FinFETs. In accordance with alternative embodiments of the presentdisclosure, gate stacks 28 are dummy gate stacks that will be replacedby replacement gates in subsequent steps. Accordingly, gate stacks 28may include dummy gate electrodes (which is also denoted as 34), whichmay comprise polysilicon, for example. Dummy gate dielectrics 32 may, ormay not, be formed between dummy gate electrodes 34 and semiconductorfins 26.

Gate stacks 28 may also include hard masks 35 and 36 formed over gateelectrodes 34. In accordance with some embodiments, hard masks 35 areformed of silicon oxide, silicon oxycarbo-nitride (SiOCN), or the like.Hard masks 36 may be formed of silicon nitride (SiN), SiOCN, SiOC, orother dielectric materials in accordance with some embodiments.

The perspective view show in FIG. 1A illustrates the exemplary layoutsof circuits 602 and 604 as schematically represented by dashed boxes. Inaccordance with some embodiments, each of circuits 602 and 604 isselected from logic circuits or SRAM circuits, and circuits 602 and 604may be same types of circuits or different types of circuits. In thefollowing discussion, circuits 602 and 604 are referred to as a logiccircuit and an SRAM circuit, respectively as an example, while othercombinations are also contemplated.

In accordance with some embodiments, circuit 602 is formed in a deviceregion including n-type FinFET region 100 and p-type FinFET region 200,and circuit 604 is formed in a device region including n-type FinFETregion 300 and p-type FinFET region 400. Device regions 100, 200, 300,and 400 are also illustrated in FIGS. 1B and 1C through 10C.Semiconductor fins 126, 226, 326, and 426 are formed in regions 100,200, 300, and 400, respectively, and are referred to collectively assemiconductor fins 26. Gate stacks 28 are formed in the directionsperpendicular to the lengthwise directions of semiconductor fins 26. Itis noted that although the gate stacks 28 are illustrated ascontinuously extending into different device regions 100, 200, 300, and400 for a compact illustration purpose, the gate stacks 28 in differentdevice regions may be physically separated from each other, or some gatestacks 28 in some of device regions may be connected in any combination,while the gate stacks 28 in other device regions are separated.

FIG. 1B illustrates the cross-sectional views of semiconductor fins 26in device regions 100, 200, 300, and 400, wherein the cross-sectionalview is obtained from the plane crossing line B-B in FIG. 1A. Also, theplane of cross-sectional view is obtained from the middle of twoneighboring gate stacks 28 (as schematically illustrated in FIG. 1C). Asshown in FIG. 1B, distance D1 between neighboring fins 126 may begreater than, equal to, or smaller than, distance D1′ betweenneighboring fins 326. Distance D2 between neighboring fins 226 may begreater than, equal to, or smaller than, distance D2′ betweenneighboring fins 426. The illustrated view in FIG. 1B reflects thestructures shown in the regions marked by dashed lines 602 and 604 inFIG. 1A (also refer to FIG. 11).

FIG. 1C illustrates the cross-sectional views of device regions 100,200, 300, and 400, wherein the cross-sectional views are obtained fromthe planes crossing lines C-C in FIG. 1A.

As shown in FIGS. 1A, 1B, and 1C, dielectric layer 38 is formed. Therespective step is illustrated as step 502 in the process flow shown inFIG. 12. Dielectric layer 38 is alternatively referred to as a spacerlayer. In accordance with some embodiments of the present disclosure,spacer layer 38 is formed of silicon nitride, silicon oxide, siliconcarbo-nitride (SiCN), silicon oxy-carbo-nitride (SiOCN), siliconoxynitride (SiON), while other dielectric materials may be used. Spacerlayer 38 may have a thickness in the range between about 2 nm and about5 nm.

Spacer layer 38 is formed as a conformal layer, and hence covers the topsurfaces and the sidewalls of semiconductor fins 26 (FIG. 1B) and gatestacks 28 (FIG. 1C). The portions of spacer layer 38 on the sidewalls ofsemiconductor fins 26 are used to form fin spacers, as shown in FIG. 2B,and the portions of spacer layer 38 on the sidewalls of semiconductorfins 26 are used to form gate spacers.

Mask layer 40 is formed over spacer layer 38. The respective step isalso illustrated as step 502 in the process flow shown in FIG. 12. Thematerial of mask layer 40 is selected to have a high etching selectivitywith relative to the material of spacer layer 38. In accordance withsome embodiments of the present disclosure, the material of mask layer40 is also selected from silicon nitride, silicon oxide, siliconcarbo-nitride (SiCN), silicon oxy-carbo-nitride (SiOCN), and siliconoxynitride (SiON). Mask layer 40 may have a thickness in the rangebetween about 2 nm and about 10 nm. Mask layer 40 is also formed as aconformal layer. The formation of spacer layer 38 and mask layer 40 maybe selected from conformal deposition methods such as Atomic LayerDeposition (ALD) and Chemical Vapor Deposition (CVD). Both spacer layer38 and mask layer 40 extend into device regions 100, 200, 300, and 400.

FIGS. 2A, 2B, and 2C illustrate the patterning of spacer layer 38 inregion 100. First, photo resist 42 is applied and patterned, wherein thephoto resist 42 is illustrated in FIGS. 2B and 2C, and not in FIG. 2A,although it still exists in FIG. 2A. Photo resist 42 may be asingle-layer photo resist, or a tri-layer photo resist including aninorganic layer (known as middle layer) sandwiched between two photoresists (known as under layer and upper layer). The patterned photoresist 42 covers regions 200, 300, and 400, and leaves region 100uncovered. Next, an etching step is performed to remove the portions ofmask layer 40 from region 100. The respective step is illustrated asstep 504 in the process flow shown in FIG. 12. Depending on the processand the distance between neighboring fins 126 (FIG. 1C), mask layer 40may or may not have a residue portion left between neighboring fins 126,as shown in FIG. 2B. After mask layer 40 is removed, the portion ofspacer layer 38 in region 100 is exposed, and an anisotropic etching isperformed to etch spacer layer 38 in region 100, so that the topportions of spacer layer 38 on top of fins 126 are removed, exposingfins 126. The respective step is illustrated as step 506 in the processflow shown in FIG. 12. The remaining portions of spacer layer 38 on thesidewalls of gate stacks 28 become gate spacers 144 (FIG. 2C), and theremaining portions of spacer layer 38 on the sidewalls of fins 126 (FIG.1B) become fin spacers 146 (FIG. 2B). The etching time of spacer layer38 is selected so that fin spacers 146 have appropriate height H1 (FIG.2B).

In a subsequent step, the exposed semiconductor fins 126 are recessed,for example, in an anisotropic or isotropic etching step, so thatrecesses 148 (FIGS. 2B and 2C) are formed to extend into semiconductorfins 126. The respective step is illustrated as step 508 in the processflow shown in FIG. 12. The bottoms of recesses 148 may be higher than,level with, or lower than the top surfaces of STI regions 22. Theetching is performed using an etchant that attacks fins 126, and hardlyattacks fin spacers 146. Accordingly, in the etching step, the height offin spacers 146 is substantially not reduced. After the formation ofrecesses 148, photo resist 42 is removed, for example, in an ashingstep.

FIGS. 3A, 3B, and 3C illustrate the patterning of spacer layer 38 inregion 300. First, photo resist 50 is applied and patterned, wherein thephoto resist 50 is illustrated in FIGS. 3B and 3C, and not in FIG. 3A,although it still exists in FIG. 3A. Photo resist 50 may also be asingle-layer photo resist or a tri-layer photo resist. The patternedphoto resist 50 covers regions 100, 200, and 400, and leaves region 300uncovered. Next, an etching step is performed to remove the portions ofmask layer 40 in region 300. The respective step is illustrated as step510 in the process flow shown in FIG. 12. Depending on the process andthe distance between neighboring fins 326 (FIG. 2C), mask layer 40 may,or may not have a residue portion left between neighboring fins 326(FIG. 3C). After mask layer 40 is removed, the portion of spacer layer38 in region 300 is exposed, and an anisotropic etching is performed toetch spacer layer 38, so that the top portions of spacer layer 38 on topof fins 326 are removed, exposing fins 326. The respective step isillustrated as step 512 in the process flow shown in FIG. 12. Theremaining portions of spacer layer 38 on the sidewalls of gate stacks 28become gate spacers 344 (FIG. 3C), and the remaining portions of spacerlayer 38 on the sidewalls of fins 326 (FIG. 3B) become fin spacers 346(FIG. 3B). The etching time of spacer layer 38 is selected so that finspacers 346 will have appropriate height H3 (FIG. 3B).

In a subsequent step, the exposed semiconductor fins 326 are recessed,for example, in an anisotropic or isotropic etching step, so thatrecesses 348 (FIGS. 3B and 3C) are formed to extend into semiconductorfins 326. The respective step is illustrated as step 514 in the processflow shown in FIG. 12. The bottoms of recesses 348 may be higher than,level with or lower than the top surfaces of STI regions 22. The etchingis performed using an etchant that attacks fins 326, and hardly attacksfin spacers 346. Accordingly, in the etching step, the height of finspacers 346 is substantially not reduced. After the formation ofrecesses 348, photo resist 50 is removed.

FIGS. 4A, 4B, and 4C illustrate the simultaneously epitaxy for formingepitaxy semiconductor regions 152 and 352 (which are source/drainregions of FinFETs) in regions 100 and 300, respectively. The respectivestep is illustrated as step 516 in the process flow shown in FIG. 12. Inaccordance with some embodiments of the present disclosure, theformation of epitaxy regions 152 and 352 includes epitaxially growingsilicon phosphorous (SiP) or phosphorous-doped silicon carbon (SiCP),and the resulting FinFETs formed in regions 100 and 300 are n-typeFinFETs. As shown in FIG. 4B, in the initial stage of the epitaxy, thegrown epitaxy regions 152 and 352 are confined by fin spacers 146 and346. After the epitaxy regions 152 and 352 are grown to higher than thetop ends of epitaxy regions 152 and 352, respectively, lateral growthalso occurs along with the vertical growth, and epitaxy regions 152 and352 expand laterally.

The portions of epitaxy regions 152 grown from neighboring recesses 148may be merged as a large epitaxy region, or remain separated from eachother when the epitaxy if finished. The portions of epitaxy regions 352grown from neighboring recesses 348 may also be merged into a largeepitaxy region, or remain separated from each other when the epitaxy iffinished. In addition, voids 153 and 353 may be formed when mergenceoccurs. Whether the mergence occurs or not depends on the heights of therespective fin spacers 146 and 346, and how long the epitaxy lasts.Accordingly, by adjusting heights H1 and H3 (FIG. 4B), one of thefollowing four scenarios may occur: the mergence occurs for both epitaxyregions 152 and 352, the mergence occurs for epitaxy regions 152 but notfor epitaxy regions 352, the mergence occurs for epitaxy regions 352 butnot for epitaxy regions 152, and the mergence doesn't occur for eitherof epitaxy regions 152 and 352. FIG. 4D illustrates some exemplaryembodiments, wherein the un-merged epitaxy regions 152 and 352 areillustrated if the corresponding epitaxy portions 608 in dashed linesdon't exist.

Referring back to FIG. 4B, if, for example, it is desirable that themergence occur for epitaxy regions 152 but not for epitaxy regions 352,fin spacers 146 may be formed to have height H1 smaller than height H3of fin spacers 346. As a result, the lateral expansion occurs earlierfor epitaxy regions 152 than epitaxy regions 352, and epitaxy regions152 merge while epitaxy regions 352 don't merge. In accordance with someembodiments of the present disclosure, to make height H1 smaller thanheight H3, the period of time TP1 for etching spacer layer 38 (the stepshown in FIG. 2B) may be selected to be longer than the period of timeTP3 for etching spacer layer 38 (the step shown in FIG. 3B). Inaccordance with some embodiments of the present disclosure, ratioTP1/TP3 may be greater than about 1.5, and may be in the range betweenabout 1.5 and about 5.0. As a result, height H3/H1 may be greater thanabout 1.5, and may be in the range between about 1.5 and about 5.0. Withthe merging of neighboring epitaxy regions, the resulting FinFET mayhave a higher drive (saturation current). With the neighboring epitaxyregions not merged, the resulting FinFETs may be more compact.Accordingly, different requirements of different circuits may be met atthe same time without requiring the epitaxy regions to be formed bydifferent epitaxy processes.

Converse to the above discussion, if it is desirable that the mergenceoccur for epitaxy regions 352 but not form epitaxy regions 152, finspacers 146 may be formed to have height H1 greater than height H3 offin spacers 346. In accordance with some embodiments of the presentdisclosure, ratio TP3/TP1 may be greater than about 1.5, and may be inthe range between about 1.5 and about 5.0. Also, height H1/H3 may begreater than about 1.5, and may be in the range between about 1.5 andabout 5.0.

In accordance with some embodiments, after the epitaxy, an implantationis performed to implant an n-type impurity such as phosphorous orarsenic into epitaxy regions 152 and 352 to form source/drain regions,which are also referred to using reference numerals 152 and 352. Inaccordance with alternative embodiments, no implantation of any n-typeimpurity is performed, and the n-type impurity was provided through thein-situ doping occurred during the epitaxy.

FIGS. 5A through 10C illustrate the formation of epitaxy regions for theFinFETs in regions 200 and 400, wherein the respective steps are similarto the repetition of the steps for forming epitaxy regions in deviceregions 100 and 300, except the conductivity types of some regions areinversed. First, an etching step is performed to remove remainingportions of mask layer 40 from regions 100, 200, 300, and 400. Therespective step is illustrated as step 518 in the process flow shown inFIG. 12. Some residue portions of mask layer 40 may (or may not) be leftafter the etch. The resulting structure is shown in FIGS. 5A, 5B, and5C.

Next, as shown in FIGS. 6A, 6B, and 6C, mask layer 56 is formed. Therespective step is illustrated as step 520 in the process flow shown inFIG. 12. The material and the formation methods of mask layer 56 may beselected from the same candidate materials and candidate methods forforming mask layer 40. For example, the material of mask layer 56 may beselected from silicon nitride, silicon oxide, SiCN, SiOCN, and SiON.Mask layer 56 may also be formed using ALD or CVD. The thickness of masklayer 56 may be in the range between about 2 nm and about 10 nm.

FIGS. 7A, 7B, and 7C illustrate the patterning of spacer layer 38 inregion 200. First, photo resist 58 is applied and patterned, wherein thephoto resist 58 is illustrated in FIGS. 7B and 7C, and not in FIG. 7A,although it still exists in FIG. 7A. The patterned photo resist 58covers regions 100, 300, and 400, and leaves region 200 uncovered. Next,an etching step is performed to remove the portions of mask layer 56 inregion 200. Depending on the process and the distance betweenneighboring fins 226 (FIG. 2C), mask layer 56 may, or may not have aresidue portion left between neighboring fins 226 (FIG. 7B). After masklayer 56 is removed, the portion of spacer layer 38 in region 200 isexposed, and an anisotropic etching is performed to etch spacer layer38, so that the top portions of spacer layer 38 on top of fins 226 (FIG.6B) are removed, hence exposing fins 226. The remaining portions ofspacer layer 38 on the sidewalls of gate stacks 28 become gate spacers244 (FIG. 7C), and the remaining portions of spacer layer 38 on thesidewalls of fins 226 (FIG. 7B) become fin spacers 246. The etching timeof spacer layer 38 is selected so that fin spacers 246 have appropriateheight H2 (FIG. 7B).

In a subsequent step, the exposed semiconductor fins 226 (FIG. 6B) areetched, for example, in an anisotropic or isotropic etching step, sothat recesses 248 (FIGS. 7B and 7C) are formed to extend intosemiconductor fins 226. The bottoms of recesses 248 may be higher than,level with, or lower than the top surfaces of STI regions 22. Theetching is performed using an etchant that attacks fins 226, and hardlyattacks fin spacers 246. Accordingly, in the etching step, the height H2of fin spacers 246 is substantially not reduced. After the formation ofrecesses 248, photo resist 58 is removed.

FIGS. 8A, 8B, and 8C illustrate the patterning of spacer layer 38 inregion 400. First, photo resist 62 is applied and patterned, wherein thephoto resist 62 is illustrated in FIGS. 8B and 8C, and not in FIG. 8A,although it still exists in FIG. 8A. The patterned photo resist 62covers regions 100, 200, and 300, and leaves region 400 uncovered. Next,an etching step is performed to remove the portions of mask layer 56 inregion 400. Depending on the process and the distance D2′ betweenneighboring fins 426 (FIG. 2C), mask layer 56 may, or may not have aresidue portion left between neighboring fins 426 (FIG. 8B). After masklayer 56 is removed, the portion of spacer layer 38 in region 400 isexposed, and an anisotropic etching is performed to etch spacer layer38, so that the top portions of spacer layer 38 on top of fins 426 (FIG.7B) are removed, exposing fins 426. The remaining portions of spacerlayer 38 on the sidewalls of gate stacks 28 become gate spacers 444(FIG. 8C), and the remaining portions of spacer layer 38 on thesidewalls of fins 426 (FIG. 8B) become fin spacers 446. The etching timeof spacer layer 38 is selected so that fin spacers 446 have appropriateheight H4 (FIG. 8B).

In a subsequent step, the exposed semiconductor fins 426 (FIG. 7B) areetched, for example, in an anisotropic or isotropic etching step, sothat recesses 448 (FIGS. 8B and 8C) are formed to extend intosemiconductor fins 426. The bottoms of recesses 448 may be higher than,level with or lower than the top surfaces of STI regions 22. The etchingis performed using an etchant that attacks fins 426, and hardly attackfin spacers 446. Accordingly, in the etching step, the height of finspacers 446 is substantially not reduced. After the formation ofrecesses 448, photo resist 62 is removed.

FIGS. 9A, 9B, and 9C illustrate the simultaneously epitaxy for formingepitaxy regions 252 and 452 (which are source/drain regions of FinFETs)in regions 200 and 400, respectively. In accordance with someembodiments of the present disclosure, the formation of epitaxy regions252 and 452 includes epitaxially growing silicon germanium, whereinboron may be in-situ doped, so that the resulting FinFETs formed inregions 200 and 400 are p-type FinFETs. As also shown in FIG. 9B, in theinitial stage of the epitaxy, the grown epitaxy regions 252 and 452 areconfined by fin spacers 246 and 446, respectively. After epitaxy regions252 and 452 are grown to higher than the top ends of epitaxy regions 252and 452, respectively, lateral growth also occurs along with thevertical growth, and epitaxy regions 252 and 452 expand laterally.

The portions of epitaxy regions 252 grown from neighboring recesses 248may be merged into a large epitaxy region. In accordance with someembodiments of the present disclosure, the portions of epitaxy regions452 grown from neighboring recesses 148 are not merged. This is achievedby making height H4 (FIG. 9B) of fin spacers 446 to be greater thanheight H2 of fin spacers 246. To make height H4 to be greater thanheight H2, the period of time TP4 for etching spacer layer 38 (the stepshown in FIG. 8B) may be selected to be shorter than the period of timeTP2 for etching spacer layer 38 (the step shown in FIG. 7B). Inaccordance with some embodiments of the present disclosure, ratioTP2/TP4 may be greater than about 1.5, and may be in the range betweenabout 1.5 and about 5.0. As a result, height H4/H2 may be greater thanabout 1.5, and may be in the range between about 1.5 and about 5.0.

In accordance with alternative embodiments, the processes for formingfin spacers 236 and 446 may be adjusted to adjust heights H2 and H4, andto result in one of the following results: the mergence occurs for bothepitaxy regions 252 and 452 (height H2 and H4 are substantially equal,for example, with difference smaller than about 10 percent), themergence occurs for epitaxy regions 452 but not for epitaxy regions 252(with height H2>H4), and the mergence doesn't occur for either ofepitaxy regions 252 and 452.

In accordance with some embodiments, after the epitaxy, an implantationis performed to implant a p-type impurity such as boron or indium intoepitaxy regions 252 and 452 to form source/drain regions, which are alsoreferred to using reference numerals 252 and 452. In accordance withalternative embodiments, no implantation of p-type impurity isperformed.

An etching step is then performed to remove remaining portions of masklayer 56 from regions 100, 200, 300, and 400, and FIGS. 10A, 10B, and10C illustrate the structure after mask layer 56 is removed. N-typeFinFET 166, p-type FinFET 266, n-type FinFET 366, and p-type FinFETs 466are thus formed in regions 100, 200, 300, and 400, respectively. Insubsequent steps, source/drain silicide regions (not shown) are formedon the top surfaces of source/drain regions 152, 252, 352, and 452. AnInter-Layer Dielectric (ILD, not shown) is formed to cover theillustrated FinFETs, and source/drain contact plugs (not shown) may beformed in the ILD to contact the source/drain silicide regions. Gatecontact plugs (not shown) may also be formed to contact the illustratedgate electrodes in gate stacks 28. Also, the illustrate gate stacks 28may be replaced with replacement gate stacks if gate stacks 28 are dummygate stacks.

The embodiments of the present disclosure have some advantageousfeatures. By separating the formation of fin spacers in different deviceregions, the heights of the fin spacers in different device regions maybe adjusted separately. This advantageously results in the flexibilityin forming merged or un-merged epitaxy source/drain regions. Theformation of the fin spacers shares a common deposition process, and theepitaxy for forming source/drain regions for different device regions isalso a common process. The manufacture cost is thus reduced.

In accordance with some embodiments of the present disclosure, a methodincludes forming a first gate stack extending on top surfaces andsidewalls of first semiconductor fins with the first semiconductor finsbeing parallel to and neighboring each other, forming a second gatestack extending on top surfaces and sidewalls of second semiconductorfins with the second semiconductor fins being parallel to andneighboring, each other, and forming a dielectric layer. The dielectriclayer includes a first portion extending on the first gate stack and thefirst semiconductor fins, and a second portion extending on the secondgate stack and the second semiconductor fins. In a first etchingprocess, the first portion of the dielectric layer is etched to formfirst fin spacers on sidewalls of the first semiconductor fins. Thefirst fin spacers have a first height. In a second etching process, thesecond portion of the dielectric layer is etched to form second finspacers on sidewalls of the second semiconductor fins. The second finspacers have a second height greater than the first height. The firstsemiconductor fins are recessed to form first recesses between the firstfin spacers. The second semiconductor fins are recessed to form secondrecesses between the second fin spacers. The method further includessimultaneously growing first epitaxy semiconductor regions from thefirst recesses and second epitaxy semiconductor regions from the secondrecesses. The first epitaxy semiconductor regions grown from neighboringones of the first recesses merge with each other. The second epitaxysemiconductor regions grown from neighboring ones of the second recessesare separate from each other.

In accordance with some embodiments of the present disclosure, a methodincludes etching a first semiconductor fin and a second semiconductorfin to form first recesses. The first and the second semiconductor finshave a first distance. A third semiconductor fin and a fourthsemiconductor fin are etched to form second recesses. The third and thefourth semiconductor fins have a second distance equal to or smallerthan the first distance. An epitaxy is performed to simultaneously growfirst epitaxy semiconductor regions from the first recesses and secondepitaxy semiconductor regions from the second recesses. The firstepitaxy semiconductor regions are merged with each other, and the secondepitaxy semiconductor regions are separated from each other.

In accordance with some embodiments of the present disclosure, a methodincludes, in a common deposition process, forming a dielectric layerincluding a first portion on top surfaces and sidewalls of firstsemiconductor fins and a second portion on top surfaces and sidewalls ofsecond semiconductor fins. In separate etching processes, the firstportion of the dielectric layer and the second portion of the dielectriclayer are etched to form first fin spacers and second fin spacers,respectively. The first fin spacers have a first height, and the secondfin spacers have a second height greater than the first height. Thefirst semiconductor fins are etched to form first recesses between thefirst fin spacers. The second semiconductor fins are etched to formsecond recesses between the second fin spacers. In a common epitaxyprocess, first epitaxy semiconductor regions are growth from the firstrecesses, and second epitaxy semiconductor regions are grown from thesecond recesses. The first epitaxy semiconductor regions merge with eachother, and the second epitaxy semiconductor regions are discrete fromeach other.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A method comprising: forming a firstsemiconductor fin, a second semiconductor fin, a third semiconductorfin, and a fourth semiconductor fin protruding higher than top surfacesof isolation regions, wherein the first semiconductor fin is close to,and have a first distance from, the second semiconductor fin, and thethird semiconductor fin is close to, and have a second distance from,the fourth semiconductor fin, wherein the second distance is equal to orsmaller than the first distance; etching the first semiconductor fin andthe second semiconductor fin to form first recesses between first finspacers; etching the third semiconductor fin and the fourthsemiconductor fin to form second recesses between second fin spacers,wherein the second fin spacers are higher than the first fin spacers;and performing an epitaxy to simultaneously grow first epitaxysemiconductor regions from the first recesses and second epitaxysemiconductor regions from the second recesses, wherein the firstepitaxy semiconductor regions are merged with each other, and the secondepitaxy semiconductor regions are separated from each other.
 2. Themethod of claim 1, wherein the second distance is smaller than the firstdistance.
 3. The method of claim 1 further comprising: forming adielectric layer covering the first, the second, the third, and thefourth semiconductor fins; and etching the dielectric layer to form thefirst fin spacers and the second fin spacers.
 4. The method of claim 3,wherein the first fin spacers and the second fin spacers are formed inseparate etching steps.
 5. The method of claim 3, wherein the first finspacers and the second fin spacers are formed to have different heights.6. The method of claim 1, wherein the first epitaxy semiconductorregions and the second epitaxy semiconductor regions are source/drainregions of Fin Field-Effect Transistors (FinFETs).
 7. The method ofclaim 1, wherein the first epitaxy semiconductor regions and the secondepitaxy semiconductor regions are p-type source/drain regions.
 8. Themethod of claim 1, wherein the first epitaxy semiconductor regions andthe second epitaxy semiconductor regions are n-type source/drainregions.
 9. The method of claim 1 further comprising: forming first gatestacks on the first semiconductor fin and the second semiconductor fin;when the first fin spacers are formed, simultaneously forming first gatespacers on sidewalls of the first gate stacks; forming second gatestacks on the third semiconductor fin and the fourth semiconductor fin;and when the second fin spacers are formed, simultaneously formingsecond gate spacers on sidewalls of the second gate stacks.
 10. A methodcomprising: forming first gate stacks on a first semiconductor fin and asecond semiconductor fin; forming first fin spacers on sidewalls of thefirst semiconductor fin and the second semiconductor fin; etching thefirst semiconductor fin and the second semiconductor fin to form firstrecesses; forming second gate stacks on a third semiconductor fin and afourth semiconductor fin; forming second fin spacers on sidewalls of thethird semiconductor fin and the fourth semiconductor fin; etching thethird semiconductor fin and the fourth semiconductor fin to form secondrecesses; and performing an epitaxy to simultaneously grow first epitaxysemiconductor regions from the first recesses and second epitaxysemiconductor regions from the second recesses, wherein when the epitaxyis finished, the first epitaxy semiconductor regions are merged witheach other, and the second epitaxy semiconductor regions are separatedfrom each other.
 11. The method of claim 10, wherein the first and thesecond semiconductor fins have a first distance, and wherein the thirdand the fourth semiconductor fins have a second distance different fromthe first distance.
 12. The method of claim 11, wherein the seconddistance is smaller than the first distance.
 13. The method of claim 11,wherein the second distance is greater than the first distance.
 14. Themethod of claim 10, wherein the first fin spacers have a first height,and the second fin spacers have a second height different from the firstheight.
 15. The method of claim 14, wherein the first height is smallerthan the second height.
 16. The method of claim 14, wherein the firstfin spacers and the second fin spacers are formed in separate etchingsteps.
 17. A method comprising: forming a first semiconductor fin, asecond semiconductor fin, a third semiconductor fin, and a fourthsemiconductor fin protruding higher than top surfaces of isolationregions, wherein the first semiconductor fin is close to the secondsemiconductor fin, and the third semiconductor fin is close to thefourth semiconductor fin; forming gate stacks to cover portions of eachof the first semiconductor fin, the second semiconductor fin, the thirdsemiconductor fin, and the fourth semiconductor fin; forming first finspacers on sidewalls of the first semiconductor fin and the secondsemiconductor fin; forming second fin spacers on sidewalls of the thirdsemiconductor fin and the fourth semiconductor fin; in a first etchingprocess, etching portions of the first semiconductor fin and the secondsemiconductor fin to form first recesses; in a second etching process,etching the third semiconductor fin and the fourth semiconductor fin toform second recesses; and performing an epitaxy to grow first epitaxysemiconductor regions from the first recesses and second epitaxysemiconductor regions from the second recesses, wherein the epitaxy isperformed until the first epitaxy semiconductor regions are merged witheach other, and wherein the epitaxy is stopped before the second epitaxysemiconductor regions are merged with each other.
 18. The method ofclaim 17, wherein the first epitaxy semiconductor regions and the secondepitaxy semiconductor regions are both n-type source/drain regions. 19.The method of claim 17, wherein the first epitaxy semiconductor regionsand the second epitaxy semiconductor regions are both p-typesource/drain regions.
 20. The method of claim 17, wherein the firstsemiconductor fin and the second semiconductor fin have a firstdistance, and the third semiconductor fin and the fourth semiconductorfin are immediate neighboring fins having a second distance greater thanthe first distance.